This application claims the benefit of German Patent Application No. 10126029.6, filed on May 28, 2001.
The present invention relates to a hydraulic lift device for an attaching device of a tractor, a self-propelled working machine or an implement having a hydraulic pressure supply device and a hydraulic tank.
WO/96 03024 describes a hydraulic lift device for an attaching device, in which each lower steering arm is controlled by a separate hydraulic cylinder. Both hydraulic cylinders are connected to the pressure supply device by a switching valve. This enables the lower steering arms to be lifted jointly. The hydraulic cylinders can be connected together to the hydraulic tank. As the lowering operation of the hydraulic cylinders is caused by their own weight, a synchronized lowering operation of both hydraulic cylinders cannot be ensured, especially when the lower steering arms are loaded differently.
DE 199 39 967 A1 describes a lift device for an attaching device of a tractor, in which each lower steering arm is controlled by a separate hydraulic cylinder. A positioning recorder produces a positioning signal representing the respective pivot position of each of the lower steering arms. A control circuit is provided to achieve synchronized movement of both hydraulic cylinders. The control circuit balances the hydraulic cylinders when the positioning recorder indicates a difference between the pivot positions of the steering arms. The electronic control employed by the control circuit is cumbersome and too expensive to apply to situations which do not require such a high degree of accuracy.
DE 87 04 278 U1 discloses a hydraulic circuit for controlling the side struts of the lower steering arms of a narrow-gauge tractor. Each lower steering arm has a double acting hydraulic cylinder which allows each arm to be pivoted laterally. The hydraulic circuit has a switching position which connects one of the hydraulic cylinders to the pressure supply and actuates the other hydraulic cylinder by displacing a pressure liquid volume. To accomplish this, the piston-end cylinder chamber of one of the hydraulic cylinders is connected to the piston rod-end cylinder chamber of the other hydraulic cylinder. The volume difference displaced when the circuit is in a first switching position causes a difference in piston travel in hydraulic cylinders of the same size. The hydraulic circuit has another switching position which allows for opposed movement of the two hydraulic cylinders. This results in different adjustment paths for each lower steering arm and which is desired only under special working conditions.
It is an object of the present invention to provide a hydraulic lift device for an attaching device which simply synchronizes the adjustment of the lower steering arms. This may be achieved by a specific advance or lagging of one hydraulic cylinder with respect to the other. It is also possible to achieve this synchronization at different lifting positions of the lower steering arms.
This object is achieved by the hydraulic lift device of the present invention. The hydraulic lift device comprises a first double acting hydraulic cylinder for actuating one of two lower steering arms. The first double acting hydraulic cylinder has a first cylinder housing, a first piston rod and a first piston. The first piston includes a first piston-end piston area and a first piston rod-end piston area. The first double acting hydraulic cylinder further includes a first piston-end cylinder chamber and a first piston rod-end cylinder chamber. The hydraulic lift device of the present invention further comprises a second double acting hydraulic cylinder for actuating the other of the two lower steering arms. The second double acting hydraulic cylinder has a second cylinder housing, a second piston rod and a second piston. The second piston includes a second piston-end piston area and a second piston rod-end piston area. The second double acting hydraulic cylinder also has a second piston-end cylinder chamber and a second piston rod-end cylinder chamber.
The hydraulic lift device of the present invention further comprises a first switching valve which is adapted to take up at least three switching positions and is comprised of four ports. The first port represents the connection to the pressure supply device. The second port represents the connection to the hydraulic tank. The third port represents the connection to the first piston-end cylinder chamber of the first hydraulic cylinder. The fourth port represents the connection to the second piston rod-end cylinder chamber of the second hydraulic cylinder.
The present invention further includes a line connecting the first piston rod-end cylinder chamber of the first hydraulic cylinder to the second piston-end cylinder chamber of the second hydraulic cylinder. The first piston rod-end piston area of the first hydraulic cylinder and the second piston-end piston area of the second hydraulic cylinder have the same size or a specific predetermined size relationship to each other.
Finally, the hydraulic lift device of the present invention further comprises a second switching valve which is arranged in line between the first piston rod-end cylinder chamber of the first hydraulic cylinder and the second piston-end cylinder chamber of the second hydraulic cylinder. The second switching valve is adapted to take up a basic position and at least two further switching positions. The second switching valve comprises a first port for the pressure supply device and a second port for the hydraulic tank. In the basic position, the connection to the pressure supply device and to the hydraulic tank, as well as to a fifth port to the second piston rod-end cylinder chamber of the second hydraulic cylinder are closed. The third and the fourth port are open to the line between the first piston rod-end cylinder chamber and the second piston-end cylinder chamber.
The present invention is advantageous because a synchronization or an advance or lagging of a hydraulic cylinder can be achieved in a purely hydraulic manner. To achieve this, the first piston of the first hydraulic cylinder, having a pressurized first piston-end cylinder chamber, pressurizes the second piston end cylinder chamber of the second hydraulic cylinder and displaces a proportional volume from the first piston rod-end cylinder chamber. When the second piston-end piston area of the second hydraulic cylinder is as large as the first piston rod-end piston area of the first hydraulic cylinder, the volume displaced from the first piston rod-end cylinder chamber will lead to an adjustment of the second piston in the second cylinder housing by the same volume. Specific measuring devices are not necessary to accomplish this synchronization. A specific advance or lagging of one of the hydraulic cylinders can be achieved as a result of the volume difference and the predetermined size relationship between the hydraulic cylinders. This can be advantageous in situations where leakages appear, as the advancing cylinder first reaches the end position in the cylinder housing when a pressure limitation is given. The other cylinder can also reach the end position to be able to correct for length differences which result from leakages.
As both cylinder chambers are filled with pressurized liquid, a further advantage is a stabilization of the lower steering arms during an eccentrically loading operation. This advantage is achieved when the first switching valve takes up the closed position. An adjustment of only one lower steering arm can also be achieved for situations when it is necessary to balance transversal inclinations or when the design of a device requires adjustment. During further normal lifting operations, the two lower steering arms continue to move in synchronization.
In a further embodiment of the present invention, concerning the first switching valve lines are provided for the passage of a pressure means. Specifically, pressure is exchanged between the first port and the pressure supply device, between the second port and the hydraulic tank, between the third port and the first piston-end cylinder chamber and between the fourth port and the second piston rod-end cylinder chamber. The first switching valve is able to take up a first switching position, in which all the ports are closed. The first switching valve is also able to take up a second switching position, in which the pressure supply device is connected to the first piston-end cylinder chamber and the second piston rod-end cylinder chamber is connected to the hydraulic tank. Finally, the first switching valve is able to take up a third switching position in which the pressure supply device is connected to the second piston rod-end cylinder chamber and the first piston end cylinder chamber is connected to the hydraulic tank.
In a further embodiment of the present invention, the second switching valve is switchable to a first further switching position, thereby (1) closing the connection to the first piston rod-end cylinder chamber of the first hydraulic cylinder, (2) connecting the second piston-end cylinder chamber of the second hydraulic cylinder to the pressure supply device and (3) connecting the second piston end cylinder chamber of the second hydraulic cylinder to the hydraulic tank. The second switching valve is switchable to a second further switching position, thereby (1) closing the connection to the first piston rod-end cylinder chamber of the first hydraulic cylinder, (2) connecting the second piston-rod-end cylinder chamber of the second hydraulic cylinder to the pressure supply device and (3) connecting the second piston rod-end cylinder chamber of the second hydraulic cylinder to the hydraulic tank.
The lower steering arms can also take up a floating position, i.e., both are freely adjustable in height relative to their pivoting position around their pivoting axis. To achieve this, the first switching valve is switchable to a fourth switching position, thereby (1) connecting the first piston-end cylinder chamber of the first hydraulic cylinder and the second piston rod-end cylinder chamber of the second hydraulic cylinder to the hydraulic tank and (2) closing the first port to the pressure supply device. The second switching valve is switchable to a third further switching position, thereby (1) closing the first port to the pressure supply device, (2) closing the fifth port to the second piston rod-end cylinder chamber of the second hydraulic cylinder and (3) connecting the first piston rod-end cylinder chamber of the first hydraulic cylinder and the second piston-end cylinder chamber of the second hydraulic cylinder to the hydraulic tank. The cylinder chambers of both hydraulic cylinders are resultingly connected to the hydraulic tank and a pressure means is exchanged.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.